1. Field of the Invention
This invention relates to modulators, and more specifically to a modulator for modulating a carrier wave with a transmission signal to be transmitted.
2. Description of the Background Art
First, a modulator disclosed in Japanese Patent Laying-Open No. 6-152675 (hereinafter referred to as "first modulator") is described in reference to FIG. 12 which shows its block structure. In FIG. 12, the first modulator is a device for modulating a carrier wave with a transmission signal to generate a modulated signal. It includes a quadrature base band generator 71, a quadrature polar coordinate converter 72, a phase accumulator 73, an adder 74, a sine wave table memory 75, a multiplier 76, and a D/A converter 77.
The quadrature base band generator 71 converts an inputted digital signal into a quadrature base band signal. When a modulated signal is represented as the synthesis of two carriers orthogonal to each other, the quadrature base band signal represents the amplitude and the phase of these carriers. The quadrature polar coordinate converter 72 converts the quadrature base band signal into a phase modulating signal and an amplitude modulating signal. When the modulated signal is represented in a polar coordinate system, the phase modulating signal and the amplitude modulating signal represent the phase and the amplitude of the modulated signal. The adder 74 adds the phase modulating signal to an output of the phase accumulator 73. The sine wave table memory 75 outputs a carrier signal of a sine wave based on an output from the adder 74. The multiplier 76 multiplies the carrier signal by the amplitude modulating signal. Thus, the first modulator generates a modulated signal with prescribed variations in phase and amplitude.
Next, a modulator disclosed in Japanese Patent Laying-Open No. 6-244883 (hereinafter referred to as "second modulator") is described referring to FIG. 13 which shows its block structure and FIGS. 14a to 14d showing the output waveforms from each of its components.
In FIG. 13, the modulator includes a signal point arranging circuit 81, a complex coefficient BPF (Band Pass Filter) 82, a latch 83, a D/A converter 84, and an analog BPF 85, and performs modulation without generating a trigonometric function.
The signal point arranging circuit 81 outputs a quadrature base band signal. The quadrature base band signal is a signal sampled at a sampling rate f.sub.c /2, and has harmonic components of an integral multiple of f.sub.c /2, as shown in FIG. 14a. The complex coefficient BPF 82 converts the quadrature base band signal into a complex band signal for selecting a prescribed frequency band. Therefore, as shown in FIG. 14b, only the components of the quadrature base band signal within the prescribed frequency band are selected. The latch circuit 83 and the D/A converter 84 multiply a real signal component of the complex band signal by a pulse whose duty ratio is smaller than 1 to perform a pulse amplitude modulation. The analog band pass filter 85 extracts a desired harmonic component from the output of the D/A converter 84. Since the D/A converter 84 converts only the real signal component, the output signal from the D/A converter 84 causes aliasing components as shown in FIG. 14C. These aliasing components are the result of the signal shown in FIG. 14b being folded at an operating frequency of the complex coefficient BPF 82, f.sub.s. The analog BPF 85 only extracts a prescribed harmonic component of the output signals from the D/A converter 84 to generate a modulated signal as shown in FIG. 14d.
As described above, the first modulator has one trigonometric function generating portion composed of the phase accumulator 73 and the sine wave table memory 75. The trigonometric function generating portion operates with a clock rate of at least more than double the frequency of the above carrier signal (the center frequency of the modulating signal). As a result, it is necessary for the first modulator to operate the trigonometric function generating portion, a complex circuit, with a frequency of at least more than double the modulating wave frequency, which disadvantageously results in an increase in circuit size and power consumption.
As in the second modulator, it is possible to construct a modulator without using a trigonometric function generating portion. However, in this case, since an analog band pass filter is required, it is not suitable for integration as a circuit. Further, in the second modulator, the center frequency of the modulating signal is limited to an integer multiple of fc/2, it cannot be changed easily.